da_stunt.R

library(dplyr)
library(tidyr)

d6 <- 1:6
# We'll call the 3 dice X, Y, and D (D is the dragon die)
all_rolls <- expand.grid(X=d6, Y=d6, D=d6) %>%
    mutate(
        ## Sum of all 3 dice; determines if you hit
        sum = X + Y + D,
        ## TRUE if this roll has doubles in it
        double = X == Y | X == D | Y == D,
        ## Number of stunt points for this roll (0 if no doubles)
        SP = double * D)

## Compare every roll against every to-hit value, and set the stunt
## points to zero for all misses
all_results <- all_rolls %>%
    crossing(data.frame(to_hit = 3:18)) %>%
    mutate(hit = sum >= to_hit,
           SP=SP * hit)

attack_summary <- all_results %>%
    group_by(to_hit) %>%
    summarize(Mean_SP = mean(SP))
print(attack_summary)